Activated sludge (organic wastes) derived from the treatment of food wastes, sewage, and the like is generated in large quantities every day. For example, about 20 million tons of food wastes and about 90 million tons of sewage activated sludge are generated every year. Such organic wastes are incinerated and landfilled and are not utilized effectively in the present circumstances. Further, incineration requires a dehydration process, which is very expensive. Also, there is a problem of landfill sites for final wastes obtained after incineration, and such sites are expected to run out in several years. On this account, there is a need for a method for effectively reutilizing or recycling organic wastes.
As an attempt to reutilize organic wastes, it is proposed, for example, that fish offal and the like is treated with at least one of supercritical water and sub-critical water so as to be converted into low molecular weight substances such as acetic acid (see, for example, JP 11(1999)-342379 A and JP 2002-18393 A). However, in order to separate, collect, and reutilize acetic acid and the like obtained in this method, it is necessary to establish a further technique, which is yet to be in practical use.
On the other hand, subjecting sewage activated sludge to methane fermentation and recovering energy also has been attempted. A reaction path from a complex organic compound such as activated sludge to methane fermentation requires the following processes (1) to (4), for example.    (1) A complex organic compound is decomposed gradually into simple organic substances by hydrolysis.    (2) Simple organic acid is made into volatile organic acid by a fermentation action of acidogens.    (3) Long chain volatile organic acid with 3 or more carbon atoms is converted into acetic acid and H2 by obligate hydrogen producing acetogens.    (4) The produced acetic acid and H2 are converted into CH4 by methanogens.
In such a conventional technique, it takes a very long time to decompose a complex organic compound such as sewage activated sludge by the action of bacteria. Further, methane fermentation generally requires two vessels, i.e., an acid production vessel and a methane fermentation vessel, in many cases. This is because unless sludge is decomposed finally into acetic acid, hydrogen, or carbon dioxide, the methanogens are not active and thus no methane is generated. In order to accelerate the decomposition of organic substances, a method including the following process is developed: a process in which sludge is pulverized into small pieces and mixed with dilution water so as to be solubilized, or a process in which an inhibitory substance in the decomposition is removed. Consequently, contact efficiency with bacteria is improved, and a reaction rate is increased. However, some organic substances cannot be decomposed into hydrogen and carbon dioxide, and the carbon digestion efficiency thereof is 20% to 50%. Thus, there is a need to treat undigested residual substances, which may require large waste water treatment equipment. Further, in the treatment of sewage sludge or dairy wastewater (cow dung, urine, and the like) using such a methane fermentation method, it is required to provide a retention time of 20 to 60 days in an actual plant and of 30 to 60 days in most cases. As described above, the conventional methane fermentation process is not practical since it is expensive and inefficient as a whole and may show a huge deficit even when electric power is sold.